72 CHEMICAL ENGINEERING
expression for power (at the runner) is P « d^ + 2.12 where d = jet diameter, inches. A more exact expression is P = n62h*d/84Qn.
Governing.-— Reaction wheels are governed by varying the gate opening. The plain cylinder gate is now generally replaced by wicket-type gates or guide vanes. Outside gate gears permit of lubrication and are used with high head wheels or with single-runner concrete spiral cases. Inside gears are submerged in the wheel pit and working surfaces must be bronze-lined. The vanes are of cast
iron or steel.
Impulse wheels are governed by needle nozzles (changing the size of the jet) or deflection nozzles (diverting some of the water from the wheel). The former method is the much more economical of water, but as stated, economy is not always important. It cannot be used without auxiliary devices, where the penstock is long and the inertia of the water great. In such cases, a sudden check of flow may actually increase rather than decrease the wheel output. Combination nozzles are often used: the first action being a deflection, followed by a gradual swinging back of the nozzle to normal positon and a gradual. closing of the jet needle.
The power required to operate gates or nozzles is so great that indirect governing is general. Hydraulic pressure is generally employed, the fluid being distributed through a valve actuated by a fly-ball governor to a cylinder which operates the final control. Large electric generators can be given adequate regulation without particular difficulty: for small direct-connected plants the governing must be worked out with some care.
Construction. — Runners of reaction turbines are of cast iron, bronze, cast steel and composite of plate steel in cast hubs. The material used is determined chiefly by manufacturing considerations. Thrust bearings for horizontal units are of the plain collar type. For vertical turbines, ball, roller or oil-pressure (step) bearings are commonly used. Impulse turbine buckets are of bronze, cast iron or cast steel: cast iron being used only for the lower heads. Nozzles are cast iron or cast steel, usually with removable bronze tips and steel needles. Interior surfaces should be polished smooth. The housing is usually cast iron, sometimes with steel-plate covers.
IDraft Tube. — By using a closed tube for the discharge of a reaction turbine, the machine may be elevated above the tail water level and the velocity of discharge water largely utilized, the water doing work by suction. The end of the tube must be submerged, and it should flare toward the end. Bends introduce considerable losses. They cannot be avoided with double runner turbines where one draft tube serves both wheels. The turbine should never be more than 20 ft. above the tail water level.
Costs.— Even excepting cases where water rights are purchased, the cost of the water wheel is only a small part of the total cost of a water power plant. Wheels plus electric generators cost in normal times from $6 to $30 per horsepower, for sizes from 10,000 hp. down to 500 hp. The form (vertical or horizontal) has little effect on cost. High heads reduce costs, especially for the smaller sizes : such heads being usually applied to impulse (tangential) wheels.
Determination of Water Available. — The discharge of a stream is ascertained by measuring the water velocity at various points of the width and at such depth as to indicate the approximate mean velocity, and by considering mean velocities